1. Field of the Invention
The present invention relates to an optical scanning device, an image forming apparatus including the optical scanning device and an optical scanning method.
2. Description of the Related Art
An optical scanning device has been widely used in a laser printer, a facsimile machine, a digital copier and other such image forming apparatuses. Recently, image forming density has been increased, and high-density image forming in which the image density exceeds 1200 dpi is being used.
In order to achieve such high-density image formation, it is necessary to sufficiently reduce a diameter of a beam spot which scans a surface to be scanned. In order to obtain a small-diameter beam spot, it is necessary to reduce the wavelength of the light emitted by a light source, and, also, to increase the NA of an optical system.
Further, in order to cause the diameter of the beam spot to be stable and uniform size, it is necessary to achieve the optical system in which the allowable degree for depth of focus is high, which system tolerates possible `component allowance (the curvature radiuses, thickness, refractive indexes) deviations` of optical elements, mounting errors, and environment variation such as variation in temperature/humidity. For this purpose, it is necessary to optimize not only the optical system on and after the optical deflector but also the optical system on the light-source side of the optical deflector.